The invention concerns a centrally fed antenna system and a process to optimize it.
Centrally fed antenna systems usually have a single reflector and a feed system, although double reflector systems are known where the feed system irradiates a subreflector that itself irradiates a main reflector. In the following, only a single reflector antenna system will be discussed; however, the designs can also be used for double reflectors.
In comparison to antennas with a single reflector and offset feed system, centrally fed antenna systems with a single reflector are more compact. In regard to the electromagnetic properties, a centrally-fed antenna does not have offset cross-polarization and hence generates less cross-polarization than an antenna system with a single reflector and an offset feed system. However, centrally-fed antenna systems have two substantial disadvantages in regard to electromagnetic properties: First, the electromagnetic field sent by the reflector is shaded by the feed system, the supports for the feed system, and the feed cable; second, this electromagnetic field affects the feed system. The shading basically influences the copolar polar antenna pattern. It produces a ripple in the pattern in the main beam direction and changes the level of the side lobes. Additional cross-polarization arises for circular polarized, centrally fed antennas. The effect on the feed system from the near field reflected by the reflector basically influences the cross-polarized antenna pattern and the reflection factor of the overall system.
The shading can be reduced by making the parts of the antenna system in the near field (that is, the supports, feed system and cable) as transparent as possible for the electromagnetic field. In addition, electrically conductive sheathing can reduce additional scatter in the near field and hence noise in the far field.
Dispersion or scatter bodies such as small cones that are placed in the centre of the reflector can reduce the effect of the near field on the feed system. The scatter bodies are shaped so that the stray field that proceeds from them and the near field reflected by the reflector destructively overlap at the feed system so that a zero area is generated at this location. This stray field of course also influences the far field as well.
The invention is based on the problem of modifying a centrally fed antenna system so that the effect of the shading and the reaction of the feed system are clearly reduced. In addition, a procedure will be presented to attain this.
The features of patent claim 1 solve these problems regarding a centrally fed antenna system. In regard to the procedure, these problems are solved by the features of the additional independent patent claims.
Basically, the entire effective reflector surface is shaped so that the maximum of the copolar far field lies on the irradiated coverage area corresponding to the requirements of the far field, and the minimum of the copolar near field lies at the feed system, e.g. at the aperture of a horn.
The actual shape of the effective surface of the reflector system is determined on a computer with a software program. First the surface of the reflector is calculated using a program according to the requirements of the copolar far field. The influences of the effect between the reflector surface and feed system can be initially ignored. There exists such a prior-art program and is generally termed a PO program, i.e., physical optics (see for example Stig Busk Sorensen: Manual for POS, Physical Optics Single Reflector Shaping Program; TICRA Engineering Consultants, Copenhagen, Denmark, June, 1995). A computer model of an antenna system adapted to the requirements of the copolar far field is obtained.
This computer model is then optimized with an optimization program that is used basically for the entire effective reflector surface so that the effects of the near field on the feed system are essentially reduced to nothing without basically changing the properties of the copolar far field.
Such a procedure that optimizes the entire effective antenna surface substantially improves the reflection factor of the entire system and the copolarization and cross-polarization properties.